Umm, they obviously did know about Opera because they explicitly redirected it to a file called error.html.
That's the key part of the story. We don't know if Opera would have ruled out the vendor simply because the administration interface didn't function properly in the Opera browser. It was that the vendor explicitly blocked the use of Opera, even though it might have worked just fine.

Lack of using standards is as much the fault of the IT people who choose products and technologies as the vendors who sell them. The number of times that IT staff either don't consider whether the product uses standards - or worse they intentionally choose proprietary solutions because they like the vendor - exceeds the imagination.
It's as much a demand problem as a supply. In this instance, the potential customer was the vendor who was being locked out so it kind of bit the manufacturer on the ass.

No, that's not true. It's very possible that some terms are enforceable and valid while other terms are not. Apple takes a risk in trying to enforce the "on Apple hardware only" clause, and I am sure even their own lawyers have cautioned against risking a judgment which states that this clause is not enforceable.
I say this as a Mac user and a promoter of Apple products. That doesn't mean I think a company should get carte blanche authority to determine how its products can be used. If you want to fully control something, don't sell it as a product on the market.

How is it that when someone expresses an opinion that they consider Apple to be worth it, that's "fanatical" but when you express the opposite it's just a reasoned difference of opinion?
Whackos are everywhere, and I don't notice any difference in the volume of devout Windows lovers who troll Mac forums than the number of Mac zealots who troll Windows forums.